Apparatus for reproducing quadraphonic sound

ABSTRACT

Apparatus for decoding four separate channels of information transduced from a medium having only two channels and presenting them on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. The realism is enhanced by a decoding system which accepts the two outputs from the medium, which may be a disc record, decodes them into four independent channels each carrying as predominant components the information contained in the originally recorded sound signals, and which includes a logic and control circuit and a &#39;&#39;&#39;&#39;front-back&#39;&#39;&#39;&#39; logic for controlling the gains of amplifiers associated with the four loudspeakers. The logic and control circuit utilizes a wave-matching technique for improving the separation of the four independent channels, and the &#39;&#39;&#39;&#39;frontback&#39;&#39;&#39;&#39; logic, which may be used independently of the &#39;&#39;&#39;&#39;wavematching&#39;&#39;&#39;&#39; logic, improves the separation between generally &#39;&#39;&#39;&#39;front&#39;&#39;&#39;&#39; and generally &#39;&#39;&#39;&#39;back&#39;&#39;&#39;&#39; signals, thereby to enhance the realism of the quadraphonic reproduction.

United States Patent [191 Bauer [1 3,821,471 [451 June 28,1974

[ APPARATUS FOR REPRODUCING Primary Examiner-Kathleen H. Claffy QUADRAPHONIC SOUND Assistant Examiner-Thomas DAmico [75] Inventor: Benjamin B. Bauer, Stamford, Attorney Agent or Flrm' spencer Olson Conn. 73 Assignee: CBS Inc., New York, NY. 55 f d f h l f.

pparatus or eco mg our separate 0 annes o m- [22] Flled 1971 formation transduced from a medium having only two [21] Appl. No.1 124,135 channels and presenting them on four loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. The real- [52] 1 179/100-4 ism is enhanced by a decoding system which accepts 100-1 TD the two outputs from the medium, which may be a [51] III". Cl. "04H 5/00 di record, decodes them into four independent [58] held of Search- 179/15 6 100-1 channels each carrying as-predominant components 179/1004 ST the information contained in the originally. recorded sound signals, and which includes a logic and control [56] References Cited circuit and a front-back logic for controlling the UN'TED STATES PATENTS 5??ilfi'iifi? fi ffiii i ii ii? fi ili' s. o n ro ICU! lize wa i i matching technique for improving the separation of 3:746:792 the four independent channels, and the front-back 7/1973 Scheiber 179/160 OTHER PUBLICATIONS Four Channels and Compatibility by Scheiber,

Audio Engineering Society Preprint, Oct.

logic, which may be used independently of the wave matching logic, improves the separation between generally front and generally back signals,

,thereby to enhance the realism of the quadraphonic reproduction.

12 Claims, 14 Drawing Figures .707Rb L8 w on 3% I;

7071?, 707/ L .7071. .707L 707R,

. I6 757 g g 3/0 b 325 7071., .707L,, I 328 330 R 1., ""r .707 R, I 240 +90 .707 W I? 707 b V .707L, I R], n, 306

302 362 3/4 3/8 I WAVE w 0 I f SHAPING 382 1 NETWORK .70 v I 7 W W AMPLIFIER 5 4; 340 7074,

354 350 342 1; .SHAP/IVG 346 374 NETWORK 5 370 344 m 707% 366 LOG/C AND 378 I 356 35 364 CONTROL CIRCUIT 4 W372 348 AMPLIFIER 368 j:

PATENTEBJHIIZB m4 31821; 471

INVENTOR. v BENJAMIN B. BAUER BY 0; MW .5 W 2 ATTORNEY PATENTEDJuuzamn 382L471 SHEET 2 BF 6 I N V ENTOR. BENJAMIN B. BAUER ATTORNEY PATENTEDmzs 1974 SiEET S 0? 6 vQm APPARATUS FOR REPRODUCING QUADRAPHONIC SOUND CROSS-REFERENCE TO OTHER APPLICATIONS This invention is related to the subject matter of the following co-pending applications, all of which are assigned to the assignee of the present invention: Ser. No. 40,5 10, filed May 26, 1970, now abandoned in favor of continuation-in-part application Ser. No. 164,675 filed July 21, 1971; Ser. No. 44,196, filed June 8, 1970, now Pat. No. 3,708,631; Ser. No. 44,224, filed June 8, 1970, now abandoned in favor of continuation application Ser. No. 251,544 filed Apr. 21, 1972, now also abandoned and a portion of the subject matter thereof incorporated in continuation-in-part application Ser. No. 328,874 filed Feb. 10, 1973; Ser. No. 8l,858,-filed Oct. 19, 1970, now abandoned in favor of continuation application Ser. No. 251,636 filed May 8, 1972; Ser. No. 112,168 filed Feb. 3, 1971; and Ser. No. 118,271, filed Feb. 24, 1971.

BACKGROUND OF THE INVENTION This invention relates to apparatus for recording and reproducing four separate channels of information on a medium having only two independent tracks, and more particularly to improved methods and apparatus for reproducing such information and presenting it on four loudspeakers to give the listenerthe illusion of sound coming from a corresponding number of separate sources. More particularly, this'invention is concerned with an improved decoder for four'channel sound recorded on a two-track medium in accordance with the method described in the aforementioned copending application Ser. No. 328,874.

The recording method disclosed in application Ser. No. 328,874 is based on an encoding function which results from passing four signals associated with the four channels of sound (which, for convenience, are identified as 1 L Rf and R for left-front, leftback, right-front and right-back, respectively) through six all-pass phase-shifting networks and thereafter combining them in appropriate proportions to produce two composite signals, L and R The encoder of the aforementioned application, illustrated in FIG. 1, has four input terminals 10, 12, 14 and 16, to which are respectively applied signals four channels of a quadru- .phonic program, L L,, R and R;, which are represented by phasors 18, 20, 22 and 24, respectively. It will be remembered from elementary considerations that a phasor generally represents a sinusoidal wave of a particular frequency, with the length of the phasor arrow portraying its amplitude and its direction representing the phase angle. In the interest of clarity of description to follow, the four phasors are shown as all being of equal amplitude and direction.

In the encoding process the positions of the individual phasors are modified by a plurality of all-pass phase-shifting networks 26, 28, 30, 32, 34 and 36 which have the capability of transmitting all of the frequencies within the range of interest (typically, the full audio range from 20 to 20,000Hz.) without change in amplitude but with a change of relative phase, which includes a reference phase-shift ill, which is a function of frequency, and adifi'erential phase-shift which may be any desired angle, typically 45 or 90. The all-pass networks 26 and 36 (hereinafter called ill-networks) 28 and 34 provide aphase-shift of II! 45, and networks 30 and 32 provide a phase-shift of II! 90. Networks having these properties being well known in the electrical engineering art, they will not here be described in detail. As indicated, the L; signalis applied to network28, the L signal is applied to both of networks 26 and 32, signal R is applied to both of networks 30 and 36, and signal R is applied to network 34.

a unity measure of signal R; after passage through network 34, and 0.707 of signal R1, after passage through network 36, to produce at output terminal 44 a second composite signal, R The signals 1 and R representing the total left and right channel signals, respectively, may be broadcast through an FM-multiplex transmitter and received with an FM-stereo receiver for subsequent decoding, or they may be recorded on a twotrack medium, such as a stereophonic phonograph record or two-track tape, for subsequent replay and decoding, all as explained in No. 328,874.

It will be observed from examination of phasor groups 46 and 48 portraying composite signals L and R respectively, that the encoder positions the phasors 1 L,,, R and R; at specific relative phase angles and amplitude relationships with respect to each other, which, as explained in the aforementioned co-pending application,-results in an excellent conventional stereophonic record, and at the same time admits of a'subsequent decoding operation to produce with a high degree of realism four separate sound channels.

As additional background for an understanding of the present invention, FIG- 2 illustrates how the signals encoded with the system of FIG. 1 are recorded on an ordinary stereophonic record 50 having a groove 52 in the surface 54 thereof cut by a recording stylus for subsequent replay in a well-known manner. By well-known convention, the left channel of a stereophonic record causes modulation of the inner groove wall, that is, the groove wall nearest to the center of the record, with the motion at 45 to the surface as portrayed by the arrow 1 and the right channel causes modulation of the other groove wall, that is, the wall furthest from the center, causing it to move at 45 to the surface, as indicated by the arrow R Referring now to FIG. 3, in which these modulations are portrayed as motions of the stylus (either recording or playing back the groove) the arrow 56 indicates the direction of groove motion as the record turns, and the oppositely directed arrow 58 consequently represents the relative direction of stylus motion with respect to the groove. It is seen that the signal L; causes the stylus to move at +45 and the signal R; causes stylus motion at 45, these motions being identical with those found in a conventional stereophonic record. Considering now the motion associated with the signal L which lags in the right channel with respect to the left channel co-pending application Ser.

by 90, it is seen that such motion is in the form of a counter-clockwise helix. Similarly, the modulation R which lags in the left channel by 90 relative to the right channel, appears as a clockwise helix. An advantage, therefore, of the encoding technique illustrated in FIG. 1 is that the right-front and left-front channels retain a theoretically infinite separation as with a conventional stereophonic record, while the left-back channel and the right-back channel appear more or less centered and somewhat dispersed between the two loudspeakers during playback. This is a very satisfactory way of displaying four-channel quadruphonic information in a two-track stereo system. A record made in accordance with the principles described above is campatible, that is capable of fully satisfactory performance on all conventional stereophonic phonographs, with the added capability of admitting to a decoding operation to convert its signals back into a four-channel program.

It will be observed that in phasor diagrams 46 and 48 of FIG. 1 the dominant signals L; and R; are both at 45 relative to the subdominant L and R components present in both; as a result of this phase relationship, the decoders described in the aforementioned copending application Ser. No. 118,271 produce output signals L, and R; which are in phase, and signals L and R,, which are also in phase with each other but displaced inphase relative to L; and R While this does not present a serious flaw in the performance of the system, it is preferable that all of the four phasors L,, L,,, R and R after decoding, be in phase. Accordingly, it is one object of the present invention to provide a system for encoding four signals into a pair of composite signals which, upon being decoded, results in four dominant signals which are all in phase with each other. Another object of the inventionis to provide a system for encoding four sound channels into two composite signals which avoids exaggeration of output signal intensity upon decoding when equal signals are applied to the side terminals (e.g., left-front and left-back) of the encoder. Another object of the invention is to provide a system including an encoder for encoding four sound channels into two composite signals and a decoder for use therewith capable of resolving ambiguities in reproduction caused by the appearance of equal signals on the two front channels, or on the two back channels. Still another object is to provide an encoder having the foregoing features and advantages while at the same time simplifying and reducing the cost of circuitry for accomplishing them.

SUMMARY OF THE INVENTION Briefly, the object of encoding four input signals into a pair of composite signals for recording on a twotrack medium which, upon being decoded, results in four dominant signals which are in-phase with each other, is attained by an arrangement of phase-shift networks and adding circuits which produces the following'relationships of the components of the respective composite signals. Designating (for convenience)the four input signals as L;, L,,, R and R, (for left-front, left-back, right-back and right-front), they are combined in such a manner that both the left and right composite signals contain subdominant signals L,, and R, in quadrature relationship, and respectively contain dominant L; and R; components which are in phase with each other and also in phase with their associated R and L,, components, respectively. In one embodiment, the L component leads the R component in the left composite signal and lags the R component in the right composite signal, and in a second embodiment the L,, component lags the R component in the left composite signal and leads the R component in the right composite signal. The composite signals in both cases, upon decoding, produce four predominant signals which are all in phase, and also have characteristics which avoids exaggeration of output signal intensity when equal signals are applied to the side terminals (e.g., left-front and left-back) of the encoder. t

In another embodiment, the'encoder comprises an arrangement of phase-shifting networks and summing circuits which again causes subdominant L and R components'to appear in quadrature relationship in both the left and right composite signals, with the L component leading the R component in the left composite signal and lagging the R component in the right composite signal, and the dominant L; and R, components to be in phase with each other, but in this case, the L component is out of phase with its associated subdominant R component while the R, component is in-phase with its associated subdominant L component. This phase relationship of the signal components in the left and right composite signals makes it possible to resolve ambiguities in situations when the sound signals are panned; that is, inserted into adjacent channels in an in-phase relationship.

Another important aspect of the invention resides in decodingapparatus which includes a matrix for decoding the composite signals to recover the four separate signals for presentation on four separate loudspeaker systems, particularly in the addition of further logic to the decoder control logic described in the aforementioned Bauer application Ser. No. l18,27l which is responsive to the composite signals produced by the lastdescribed encoder to distinguish betweenfront and back signals and to promptly and automatically adjust the gain of the front and back loudspeakers to enhance the realism of four-channel reproduction.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantages of the invention, and a better understanding of its construction and operation, will be evident from the following detailed description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram of the encoder described and claimed in co-pending application Ser. No. 44,224, to which reference has been made in discussing the background of the invention;

FIGS. 2 and 3, to which reference has already been made, illustrate the operation of the encoding apparatus of FIG. 1;

FIG. 4 is a pair of phasor diagrams useful in explaining the principles of the present invention;

FIG. 5 is a schematic diagram of encoding apparatus embodying the invention;

FIG. 6 is a schematic diagram of decoder apparatus described in co-pending application Ser. No. 1 18,271, useful in explaining the efficacy of the present encoding technique;

FIG. 7 is a schematic diagram of an alternative form v of encoding apparatus embodying the invention;

FIG. 8 is a schematic diagram of still another alternative form of encoding apparatus embodying the invention;

ing signals encoded with the encoder of FIG. 8;

FIG. 12A is a schematic diagram of a modification of the system of FIG. 12; and

FIG. 13 is a schematic diagram illustrating how four original sound channels may be produced for recording and reproduction in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT The encoders to be described herein are also described in the aforementioned application Ser. No. 328,874, and the disclosure thereof is incorporated herein by reference; however, the description to follow, particularly in view of the illustration of this previous encoder in FIG. 1, is believed to be sufficiently complete to enable one skilled in the art to understand its operation without recourse to the co-pending application.

The present invention is based on the discovery, illustrated in FIG. 4, that while it is important in the encoding process to maintain a 90 relationship between the phasors 0.707L and O.707R the relative positions of the phasors L, and R; may be arbitrarily chosen insofar as decoding is concerned. In other words, any decoder designed to decode the composite signals L and R in FIG. 1 (such as the decoder described and claimed in co-pending application Ser. No. l l8,27l will also satisfactorily decode the signals L and R shown in FIG. 4, regardless of the size of the angles'a and (1 between phasors L; and 0.707L and between phasors R; and 0.707R respectively. Since the ozs in FIG. 1) are established by ill-networks 28 and 34, by suitable design of these networks it is possible to place phasors L, and R at any desired position with respect to the other two phasors in the group. An especially beneficial relationship is established by making both the angles a 90 so that in the phasor group portraying L the phasor L, coincides with phasor 0.707R and in the other phasor group, the phasor R; coincides with phasor O.707L,,. This relationship is readily obtained by modifying ill-networks 28 and 34 so that instead of providing a phase-shift 1!: 45 they provide a phase-shift (I! 90. Moreover, as one of the benefits of this invention, the above relationship permits the encoding function to be performed with only four ill-networks, instead of the six required in the encoder of FIG. 1.

Referring now to FIG. 5, the encoder in accordance with the invention has four input terminals 60, 62, 64 and 66 to which input signals L L R and R; originating with the quadruphonic program, and represented by phasors corresponding to the same signals depicted in FIG. 1, are respectively applied. Rather than being applied directly to a ill-network as in the system of FIG. 1, input terminals and 64 are connected to a summing junction 68 which is operative to add a unity measure of signal L, to .707 of signal component R Similarly, terminals 62 and 66 are connected to a second summing junction 70 which is operative to add a unity measure of signal R, to 0.707 of signal L Terminal 62 is also connected to the input of ill-network 72 which introduces a relative phase-shift of II! to the signals L,,, and terminal 64 is connected to the input of a second (ab 0) network 78. The outputs of summing junctions 68 and 70 are respectively applied to the input tenninals of ill-networks 74 and 76, both of which introduce a relative phase-shift of 111 90.

The full output of ill-network 74 is added in summing junction to 0.707 of the output of network 72, and similarly, the full output of network 76 is added at summing junction 82 to 0.707 of the output of network 78. As a consequence of this phase-shifting and combining of signals, there appears at output terminal 84 a composite signal L depicted by phasor group 88, and at output terminal 86 a composite signal R depicted by phasor group 90. It will be observed that there is a oneto-one correspondence between-phasor groups 88 and 90 and the corresponding phasor groups in FIG. 4 if the angle a in the latter group is set at 90, which is in accordance with the teaching of this invention.

That the encoder of FIG. 5 is compatible with decoders intended for use with the signals encoded in accordance with the system of FIG. 1 is demonstrated by the comparative analysis presentedin FIG. 6, as applied to the decoder described in co-pending application Ser. No. 1 18,271. This decoder includes a pair of input terminals and 102 to which composite signals L and R are respectively applied. The signal applied to terminal 100 is applied in parallel to and phase-shifted by a pair of ill-networks 104 and 106, and the composite R signal applied to input terminal 102 is applied in parallel to ill-networks 1 08 and 110. These ill-networks are of the type previously described, theflnetworks 104 and l 10 introducing a phase-shift of (ill 0) and networks 106 and 108 introducing a phase-shift of (lll' 90). It will be noted that the reference angle is designated ll! instead of ill, as used in the encoder; this is to call attention to the fact that the reference phase-shift in the decoder need not be the same as in the encoder, provided the same reference phase-shift is used in all four of ill-networks 104, 106, 106 and 110. The outputs of networks 104 and 110 are applied directly to'the left-front output terminal 1 l2 and to the right-front output terminal 114, respectively. Equal portions of the outputs of networks 106 and 110 are summed in a summing junction 116, the output of which is applied to the left-back output terminal 118, and equal portions of the outputs of networks 104 and 108 are summed in a second summing network 120, the output of which is applied to the right-back output terminal 122.

A comparison will now be made of the performance of the decoder of FIG. 6 in response to signals encoded in accordance with co-pending application Ser. No. 328,874, and to composite signals encoded in accordance with the present invention. Phasor groups corresponding to signals encoded in accordance with application Ser. No. 328,874 are shown in dotted lines, and the phasor groups encoded by the encoder of FIG. 5

are shown in solid lines. Phasor groups and 132 portray the two input signals I and R which, upon being shifted in phase by the all-pass networks 104, 106, 108 and 110 appear as new phasor groups 134, 136, 138 and 140. The phasors in these latter four groups are labeled with a prime-to differentiate them from the corresponding phasors prior to'introduction of the relative phase-shifts. The signal represented by phasor group 134 appears at output terminal 112 as 7 phasor group 142 and contains a dominant component L, together with the smaller components O.707L and 0.707R The phasor groups 136 and 140 after summing in junction 116 result in a signal at output terminal 118 represented by phasor group 144 containing a dominant phasor L and subsidiary phasors 0.7071 and 0.707R The sum of phasors 134 and 138 appearing at the output of summing junction 120 (output terminal 122) is a composite signal represented by phasor group 146 having a dominant phasor R accompanied by subsidiary signals 0.707R, and 0.707L,. Finally, the phasor group 140 appears at output terminal 114 as phasor group 148, and contains a dominant signal R,

together with subsidiary signals 0.707R and O.707L,,. Thus, the decoded signals appearing at output terminals 112, 118, 122 and 114 each contains its appropriate dominant signal together with signals from two other channels diminished by the factor O.707.-It will be noted that the two principal front channel phasors, namely L, in group 142 and R, in group 148 are in phase, and that the two principal back channel vectors, L,, in phasor group 144 and R, in group 146, are also in phase with each other, but not in phase with the L, and R, phasors. While this phase relationship does not represent a major flaw in the performance of the system, it has been found preferable that the four predominant phasors all be in phase.

It will now be demonstrated that this favorable phase relationship is achieved when signals'encoded with the encoder of FIG. 5 are decoded in the decoder of FIG. 6. It will be remembered from the description of FIG. 5 that the encoded signals L and R are as portrayed by phasor groups 150 and152, the former after being acted upon by ill-networks 104 and 106 appearing as phasor groups 154 and 156, respectively, and the R signal after passage through Ill-networks 108 and 110 appearing as phasor groups 158 and 160, respectively. Phasor groups 154 appears at output terminals 112 as phasor group 162, and phasor group 160 appears at output terminal 114 as phasor group 164. These output signals contain predominant signals L, and R,, respectively. Phasor group 154 appears at output terminal 112 as phasor group 162, and phasor group 160 appears at output terminal 114 as phasor group 164.

These output signals contain predominant signals L, and R,, respectively, which are in phase with each other, and each includes subsidiary signals O.707R and 0.707L

The phasor groups 156 and160 upon being summed in summing junction 116- produces at output terminal 118 the composite signal portrayed by phasor group 166, and the sum of the signals represented by phasor groups 154 and 158 appearing at the output terminal 122 of summing junction 120 is as portrayed by phasor group 168. It will be noted that phasor groups 166 and 168 contain predominant phasors L and R respectively, which are in phase with each other, and also in phase with the predominant phasors in groups 162 and 164, and each accompanied by two-subsidiary signals 0.707R/ and -O.707L Comparison of phasor groups 162 with 142, 166 with 144, 168 with 146, and 164 with 148 reveals that they contain the same respective subsidiary signals in the same magnitude and in the same intergroup phase relationships. Therefore, the respective signals will be capable of properly activating the enhancing logic and control circuits described in co-pending applications Ser. Nos. 328,874 and Another advantage of the present encoding technique will be seen from a comparison'of phasor groups 144 and 166, for example, in each of which there is shown in dotted line a signal L, which results from applying equal signals to the left-front and left-back terminals of the encoder. Because of the angular relationship between phasors 0.7071 and O.707L in phasor group 144 as compared to the guadrature relationship between the corresponding phasors in group 166, the resulting phasor L in group 144 is of greater magnitude than the corresponding phase. in group 166. The lack of exaggeration of the signal L, is of significant advantage, and by symmetry, it will be recognized that exaggeration of the R signal which would result from application of equal signals to terminals 64 and 66 of the encoder of FIG. 5 is likewise avoided.

In summary, the improved encoder of FIG. 5 offers three significant advantages over the encoder of FIG. 1: 1) it provides encoding with four, instead of six ill-networks, with an attendant reduction in the cost of the encoder; 2) it produces encoded signals which, upon decoding, cause the predominant signals to all be in phase; and 3) it avoids exaggeration of output signal intensity from the decoder when equal signals are applied to the side terminals of the encoder.

FIG. 7 illustrates a modification of the encoder of FIG. 5, differing therefrom in the manner in which the four input signals are added and phase-shifted. In this case, the full L; signal applied at terminal 170 is added to 0.707 of the R signal applied to input terminal 174 in a summing junction 178, and the full R, signal applied at input terminal 176, is added in summing junction 180 to 0.707 of the L, signal applied at input terminal 172. The sum signals from summing junctions 178 and 180 pass through respective til-networks 182 and 184 and are added in respective summing junctions 186 and 188 to 0.707 of signals L and R,,, respectively, after being shifted in phase by II! 90 in respective til-networks 190 and 192. The L and R signals appearing at output terminals 194 and 196, represented by phasor groups 198and 200, respectively, are similar to the corresponding phasor groups 88 and 90 in FIG. 5 except that in group 198 the 0.707R phasor leads the O.707L,, phasor, wherein in group 88 the L phasor v leads the'R phasor; the positions of the L and R phasors in groups 200 and are similarly interchanged. While this decoder provides perfectly consistent signals, it has the slight disadvantage stemming from the fact that the 0.707R phasor in group 198 leading'the corresponding signal in phasor group 200 tends to cause this right-back signal to appear to lean slightly toward the-left-front channel when the record is replayed stereophonically over two loudspeakers. By symmetry, the left-back signal likewise will tend to lean slightly to the right when the record is replayed stereophonically. Thus, while the alternative encoder of FIG.

7 produces acceptable composite signals for reproduction over four loudspeakers, it is inferior to the encoder of FIG. 5 if the record carrying the encoded signals is to be played over a two-channel stereophonic playback apparatus.

Referring again to FIG. 4, although the values of angles a, and er of 90 are preferred, it may be desirable in some cases to chose other values, for example, 0 or 180, or a, and a may be different, as will now be obvious to those skilled in the art.

By seemingly slight modifications of the encoder of FIG. 7, and of the decoder shown in FIG. 6 (which is fully described and claimed in the aforementioned copending application, Ser. No. 118,271, the performance of the overall system can be significantly improved, particularly in its ability to resolve ambiguities in cases when the sound signals are panned; thatis, inserted into adjacent channels in an in-phase relationship. The cause of such ambiguities will become apparent from analysis of the decoded signals delivered by the decoder of FIG. 6 which, it will be remembered, contain predominant signals L L,,, R; and R respectively, together with two contaminating signals from other'channels. Actually, the contaminating signals are not noticed when all four predominant signals are simultaneously present, as when four different performers produce four parts of a musical selection in all four channels, since there is sufficient mixing of sound in the room or listening area that the presence of the contaminating signals in the individual channels is inconsequent'ial. They are noticable, however, when sound is present in only a single channel, or in at most two channels, because in these instances, when the sound should be coming from a single loudspeaker or from two loudspeakers, it is instead heard from all four, which is readily noticable and sometimes objectionable. This situation is improved, and the realism of four channel reproduction enhanced by the logic and control systems described in the aforementioned co-pending applications Ser. No. 328,874 and 1 18,271 which recognize the presence of sounds in individual channels and generate signals for controlling the gain of gain control amplifiers in the individual loudspeaker circuits in response to the instantaneous presence of the predominant signals. Thus, if a signal appears only in the left-front channel, for example, (and which, because of the protocol of the decoder, also appears at reduced level in both of the back channels) the logic functions to enhance the gain of the front loudspeaker amplifiers and to turn down the gain of the back loudspeaker amplifiers thereby to cause the sound to appear to-originate at the left-front loudspeaker only. The logic and control circuitry operates similarly with respect to the other three loudspeakers with the consequence that when artists are performing in concert in all four channels the gain of the respective amplifiers are increased and decreased to instantaneously enhance the channel or channels in which signals are predominant at a particularly instant to give a highly realistic replication of the original four channel program.

The above-described methods ofencoding four signals into two and decoding them back into four works very well in the majority of circumstances, one exception, however, being when the sounds are panned, that is, inserted into adjacent channels in an in-phase relationship, and then only in two specific instances: namely, when the sound is panned precisely in the middie of the two front channels by application of equal signals to the L; and R, encoder terminals, or when it is panned precisely in the middle of the two back channels L, and R,,. It can be shown that in these two circumstances the modulation produced on the stereophonic disc is the same when the two front channels are panned as it is when the two back channels are panned. Consequently, the logic and control system used with the decoder is unable to distinguish whether such panned sound signal belongs to the front channels or to the back channels, resulting-in an ambiguity. In accordance with another aspect of the present invention this ambiguity is resolved by modification of the encoder and the decoder, thereby to provide a significant improvement in performance of the system.

The modified encoder, illustrated in FIG. 8, has four input terminals 210, 212, 214 and 216 to which the four signals L,, L,,, R), and R,, depicted as in-phase signals of equal amplitude, are respectively applied. The total L; signal is added in a summing junction 218 to 0.707 of the R signal, the output of the summing junction being applied toa phase-shifting network 220 which introduces a reference phase-shift 111, which is a function of frequency. The full R, signal at terminal 216 is added in summing network 222 to 0.707 of the L signal appearing'at input terminal 212, and the output is passed through the (II-network 224, which also provides the reference phase-shift ill. The L, and R signals are also applied to respective lll networks 2'26 and 228, each of which provides a phase-shift of 1!; The full signal appearing at the output of network 220 is added in a summing circuit 230 to 0.707 of the signal appearing at the output of network 226 to produce at its output terminal 232 a composite signal designated L Similarly, the full signal from network 224 is added in summing junction 234 to 0.707 of the signal from network 228, the latter in this case being in the positive sense. The signal appearing at the output terminal 236 is the composite signal designated R As in the case of the other encoders, the signals L and R? may be transmitted by FM multiplex radio, or they may be recorded on any two-channel medium such as a two-track tape or stereophonic record-for later reproduction.

The significance of the modifications to the encoder of FIG. 7 to provide the encoder of FIG. 8 (namely, the reversal of the phase of the 0.707 terminals of the two summing circuits in the upper half of the diagram) will be appreciated from an analysis of the phasor relationship of the L and R composite signals portrayed as phasor groups 238'and 240, respectively. It will be observed that phasor group 238 consists of the signal L; (which although shown in the same phase relationship as the input signal L, has a rIJ-as-a-function-offrequency angle difference between them), a signal 0.707R, in a negative sense with respect to its corresponding input phasor, and a 0.707L signal which lags phasor 0.707R by 90 because of the action of network 226. Phasor group 240 consists of the original signal R, in the same relative phase position as its corresponding input signal, a signal 0.707L in phase with the R; signal, and a .707R signal lagging the 0.707L signal by 90 due to the action of til-network 228. As has been pointed out hereinabove, in the interest of providing better realism of image placement when the record is played in conventional stereophonic mode over two loudspeakers, it is preferable to arrange the phasor 0.707L, in phasor group 240.to lag behind the similarly numbered phasor in phasor group 238, and conversely, to arrange the phasor 0.707R in phasor group 238 to lag behind the corresponding phasor in group 240. Thus, the connections shown in FIG. 8 are preferred, but it is possible to interchange the phase positions of the phasors and still obtain the principal benefits 0f the invention.

With reference again to FIG. 2, when the encoded signals represented by phasor groups 238 and 240 are applied to a stereophonic photograph record, the left groove is modulated during the cutting process in the direction of the arrow L (which is at 45 to the surface of the record) under the influence of the left-channel signal L while the groove is modulated in the direction of the arrow R under the influence of the rightchannel signal R Referring now to FIG. 10, the effect of the panning is to divide the signal (as by means of two coupled attenuators) between two channel inputs. At the midpoint of the panning operation, the signal becomes precisely divided between the front channels L, and R or between the back channels L or R this condition will now be examined. The phasor groups 238 and 240 from FIG. 8 are repeated here as phasor groups 250 and 252, respectively, and the panned center signals have been added. The front center signal, C is placed in the proportion 0.707C, andin-phase in the phasor 7 groups 250 and 252, appearing as phasors 254 and 256.

Since these phasors are equal and in-phase they cause the arrows L and R in FIG. 2 to combine to produce a horizontal motion; accordingly, the center front signal C appears as a horizontal arrow 246 in FIG. 9. It is seen, thus far, that the phasor group in FIG. 9 depicts the left-front channel phasor L;, the center channel phasor C;, and the right-front channel'phaso'r R in a relationship which those skilled in the art will recognize as portraying the modulation of a conventional stereophonic record. 7

Reverting now to FIG. 10, it will be noted that the center-back channel C is divided in the proportion 0.707 in the left back and right back channels, and since these two phasors appear as a 0.707 fraction, the

corresponding fraction of the C, signal is 0.5 in phase with the 0.707L phasor and 0.5 in phase with the 0.707R phasors in both phasor groups. With this convention in mind, it is seen that the two phasors in each group add to the larger phasors 0.707C in each of the phasor groups 250 and 252; however, it should also be observed that the phasor 0.7O7C, in phasor group 250 is out-of-phase with the corresponding phasor in group 252. This is an important quality of the encoder of FIG. 8 because now the centerback signal C is of an entirely different character than the center front signal C It will be recognized that the signal C having an out-ofphase relationship in the two channels will result in a vertical modulation of the groove 52 in FIG. 2, which is depicted by the arrow 248 inFIG. 9. It will be realized that any signal recorded in this manner cannot be reproduced by a monophonic phonograph pick-up, nor by the monophonic section of an FM multiplex transmittingstation; consequently, when using the encoder of FIG. 8 the center-back location should preferably be used for occasional sounds such as reverberation, motion during planning, etc., and not for the placement of an important artist since he would not be heard when the signal is broadcast over AM radio or over monophonic FM radio. Such signals would, however, be fully audible with stereophonic or quadruphonic modes of reproduction, and all other locations of the artist would be transmitted satisfactorily.

Another significant feature of the encoder is illustrated by the phasor groups 256 and 258 in FIG. 11, the former depicting the situation which results when the phasor groups 250 and 252 of FIG. 10 are added and the latter depicting the situation when the composite signal R (phasor group 252) is subtracted from L (phasor group 250). It will be noted that when L and R are added the phasors 1 L R, and R, all have an intensity equal to unity, whereas the front center signal C, is augmented by a factor 1.414, which is exactly what happens when a stereophonic record is played over a monophonic player. The back center signal C,,, is cancelled, however, because of the aforementioned out-of-phase relationship. When the phasor groups are subtracted, the phasors L L,,, R, and R, again all appear with unity amplitude, but this time the center back signal, C is augmented by the factor 1.414 while the center front signal, Cf, is cancelled. The relationship portrayed by phasor groups 256 and 258 are extremely important since they indicate that if only a center front signal is present, i.e., no center back signal, the phasor group 256 will be greater than group 258, and, conversely, if there is only a center back signal but no center front signal,the phasor group 258 will be the larger. This interesting property is used toadvantage to enhance the operation of the decoder to be utilized with the encoder of FIG. 8, which will now be described.

' The decoder, illustrated in FIG. 12, is in many respects similar to the decoder of FIG. 6. The signals L and R represented by phasor groups 238 and 240, respectively, are applied to respective input terminals 300 and 302, from whence they are applied in parallel to respective pairs of ill-networks 304, 306, 308 and 310. In this manner, each of the signals L and R passes without relative phase-shift through networks 304 and 308, respectively, and also pass with a relative phase-shift of through networks 306 and 310. In the phasor groups 312, 314, 316 and 318 portraying the output signals from networks 304, 306, 310 and 308, respectively, the individual phasors, essentially identical with the corresponding phasors in groups 238 and 240, are differentiated with a prime to indicate that they have been subjected to the action of a lII-n6tWOIk and thus differ from the input phasors by an angle which is a ill-function of frequency, in addition to the differential angle introduced by the networks.

The outputs of networks 304 and 308 are applied directly to the input terminals of respective gain control amplifiers 3'12 and 314, the outputs of which are applied to respective loudspeakers 316 and 318. The sig- 'nals applied to loudspeakers .316 and 318 contain predominant original signals L, and R,, respectively, the

two subdominant contaminating signals 0.707L,,

and O.707R Equal proportions, namely, 0.707, of the outputs of networks 306 and 308 are summed at a sum-' ming junction 320 to produce a composite signal consisting of a predominant signal L which is applied to a gain control amplifier 322 and thence to loudspeaker 324. Equal negative portions, namely, 0.707, of the outputs of networks 304 and 310 are summed at a second summingnetwork 326 to produce for application to a fourth gain control amplifier 328 a composite signal composed of a dominant signal, R together with 0.707R, and 0.7071 after amplification, this composite signal is applied to loudspeaker 330. It will be observed from the phasor groups 332, 334, 336 and 338 which respectively portray the composite signals appearing at loudspeakers 316, 324, 330 and 318, that the predominant phasors at all four loudspeakers are in-phase.

As has been explained previously, the contaminating signals in each of the composite signals have little effect on the listening quality of the decoder as long as all of the predominant signals are simultaneously present because there is sufficient confusion and mixing in the air of sounds from different sources that the precise location of each sound is not easy to determine. However, if the nature of the original program signals are such as to be best presented over one or two of the loudspeakers, it becomes desirable to enhance or sharpen the realism of reproduction. This is accomplished with a logic and control circuit 340 which preferably is of the type disclosed in detail in -co-pending application Ser. No. l 18,271, the teaching of which is hereby incorporated by reference. The logic described in this co-pending application is characterized as wave-matching logic which makes an instantaneous comparison of the waveshape of the signals and makes a decision to either increase the gain of amplifiers 312 and 314 which feed the two front loudspeakers 316 and 318 and diminishthe gain of the amplifiers which supply the back loudspeakers, or, conversely, whether to increase the gain of the back amplifiers 322 and 328 and reduce the gain of amplifiers 312 and 314. The information on which the logic circuit bases its decisions is derived from the outputs of ill-networks 308 and 306 and is applied to input terminals 342 and 344, respectively. The logic and control circuit 340 is operative to produce output signals at its output terminals 346 and 348 as follows: If a single channel signal L;, or two uncorrelated signals L; and Rare present in the input, the logic 340 produces output signals which are operative to increase the gain of amplifiers 312 and 314 and to decrease the gain of amplifiers 322 and 328. If, on the other hand, the principal signals are L 'alone, or uncorrelated signals L,, and R the logic is operative to produce signals which increase the gain of amplifiers 322 and 328 and to diminish the gain of amplifiers 312 and 314. However, as has been explained above, if the signals L, and R, are equal and in-phase, or, conversely, if the signals L,, and R are equal, in-phase signals, the logic described in the aforementioned co-pending application Ser. No. 1 18,271 lacks the information necessary to determine which set of amplifiers is to be turned on and which is to be turned off, resulting in a spread of the center signals toall four loudspeakers. It should be emphasized that this does not produce an unpleasant reproduction, but for the sake of greater realism in more sophisticated decoder equipment it is desirable that the gain of the front and back loudspeakers be promptly and automatically adjusted in response to the center front and center back signals. This desired action is made possible bythe encoding technique described in connection with FIG. 8, and from the recognition'that differentiation between back and front center signals, illustrated in FIG. 11, as achievable when the signals are encoded in this way.

The decoder of FIG. 12 is given this additional capability by the provision of logic in addition to that provided by logic circuit 340. The input signals L and R at input terminals 300 and 302 are applied via conductors 350 and 352, respectively, to a summing junction 354 and to a subtracting junction 356. The sum signal appearing at the output of summing junction 354 being the sum of L and R is as depicted by phasor group 256 in FIG. 11, and the output of subtracting junction 356 is a composite signal having the properties portrayed by phasor group 258. It will be evident from reconsideration of FIG. 11 that if the front center signal predominates the output of summing circuit 354 will exceed the output of subtraction circuit 356, and, conver'sely, if the back center signal predominates, the output of the subtracting junction will exceed the output of the summing junction. In order to obtain the relative ratio between these two quantities, the outputs of junctions 354 and 356 are amplified in respectivequasilogarithmic amplifiers 358 and 360 and then rectified by respective rectifiers 362 and 364, which are preferably full-wave rectifiers, and integrated with leaky integrators 366 and 368, respectively. The difference of the of outputs of the integrators, appearing at terminals 370 and 372, respectively, is proportional to the relative magnitudes of the sum and difference signals produced by junctions 354 and 356. Inasmuch as the output at terminal 370 arises from the sum of L and R while the output at terminal 372 arises from the difference of these signals, the former will be greater than the latter with the presence of a front-center signal. To make the decision that a front-center signal is present,

a subtracting junction 372 is provided which subtracts the signal at terminal 372 from the signal appearing at terminal 370. The output of subtracting junction 372 is applied to a summing junction 374 where it is added to the output signal at "terminal 346 of logic and control circuit 340, the output of junction 374 being applied in parallel to the gain control electrodes of amplifiers 312 and 314 (which supply the front loudspeakers 316 and 318) to augment their gain. A second subtracting junction 376 subtracts the signal appearing at terminal 370 from the signal appearing at terminal 372, which produces a negative output whena center-front signal is present; this is combined in a summing junction 378 with the output at terminal 348 of logic circuit 340, the sum signal serving to diminish the gainof amplifiers 322 and 328 which supply the rear loudspeakers 324 and 330, respectively. Conversely, if a center-back signal is present, the just-described logic would be operative to partially or completely turn off the front loudspeakers and to augment the gain of the rear loudspeakers.- g

It will be observed that withthe logic of'FIG. 12 the control action is derived from the sum of a pair of control signals. It may be advantageous, and it is possible by modification of the system of FIG 12, to instead control the action of the gain control amplifiers by the stronger of the control signals rather than their sum. To this end, the circuit of FIG. 12 is modified as shown in FIG. 12A (which illustrates only the portion of the FIG.

12 circuit alfectedby the modification) so as to combine the control signals from the wave-matching logic 340 and from the just-described center frontcenter back logic in an OR circuit, rather than summing them. Since the output signal at terminal 348 of the logic and control circuit 340 is simply the inverse of the signal at terminal 346, only the latter need by used; similarly, the output signal from summing junction 376 being the inverse of the output signal from junction 372, only the latter need be used.

The OR circuit takes the form of two pairs of rectifiers 380 and 382, and 384 and 386, so connected that one of the rectifiers in each pair is forward conducting and the other is backward connecting. The signal appearing at terminal 346 is applied to rectifiers 380 and 382 and the output signal from summing junction 372 is applied to rectifiers 384 and 386. By reason of a'connection therebetween the outputs of rectifiers 380 and 384 are added at points 388 and 392, and will be the greater of positive voltages at terminal 346 or at the output of junction 372. At the same time, the outputs of rectifiers 382 and 386 are combined at points 390 and 394 and will be the greater of negative voltages at terminal 346 or junction 372. The voltages appearing at points 388 and 390 are added at summing junction 374; consequently, the output of summing junction374 will be the greater of the outputs of either the wavematching logic or the center front-center back control logic, and not the sum as in FIG. 12. The negative voltage outputs at points 392 and 394 are added at summing junction 378, thereby producing the inverse of the output of junction 374. The outputs of the junctions 374 and 378 are applied to the pairs of gain control amplifiers 312 and 314 and 322 and 328, respectively, as shown in FIG. 12.

It will be recognized from the foregoing description that the basic decoder of FIG. 12 is the same as that described in FIG. 6 with the single exception that the adding junction 326 inverts the phase of the R signal; alternatively, this phase reversal action may be obtained in the amplifier 328, or in its associated loudspeaker.

If desired, the composite input signals L and R may be shaped by frequency-dependent wave-shaping networks 380 and 382, respectively, before application to the logic circuitry so as to limit the action of the logic to voice signals, for example, or other instrumental sound, to make the logic less susceptible to erroneous decisions with certain high power, low frequency instruments, such as the bass drum.

There are obvious precautions which should be taken to obtain best results from the encoder of FIG. 8 and the decoder of FIG. 12. For example, it is not advisable to record an important signal, such as a soloist, in the center-back because the signal will disappear or be greatly attenuated when played over a monophonic phonograph. However, it is perfectly acceptable to record auxiliary signals, such as the sound of a vehicle moving around the room, or the sound of a symphony orchestra in a'reverberant concert hall, as diagrammatically illustrated in FIG. 13. In this example the musical groups, arranged in the front of a concert hall represented by the dashed-line enclosure 390, are designated by the dotted ellipses 400-412, the sounds produced by these groups being picked up by microphones 416-428, respectively. The microphone circuits are isolated from each other by a plurality of buffer amplifiers 430-444, connected as shown. Microphones 416, 418 and 420 are connected together and to the left front terminal 450, and microphones 424, 426 and 428 are likewise connected together and to the right front terminal 456. The output of the center microphone 422, which may be reserved for the soloist, is fed in parallel to equal-gain amplifiers 436 and 438 so as to feed equal parts of the signal to the left front and right front terminals, thus constituting the center-front channel of the record.

Reverberation and other space effects may be conveniently picked up by a suitable arrangement of a dual cardioid microphone, exhibiting polar patterns depicted by 446 and 448, placed near the center of the hall to provide a realistic reverberation delay. The outputs f the two transducers of the microphone are fed to terminals 452 and 454 which correspond to the back channels. This arrangement of microphones produces two correlated signals: the sound produced by the soloist which is applied to the front channels 450 and 456, and that due to reverberation picked up by microphone sections 446 and 448 and applied directly and in-phase to the terminals 452 and 454. Since during the performance, the orchestra and the soloist will produce the stronger signals, the output of the summing junction 354 (FIG. 12) will exceed that of the subtracting junction 356, resulting in an increase in the gain of the front loudspeakers. However, during prolonged pauses, the sound of the orchestra will decay quickly, while the reverberant sound will continue for some time. During this time the reverberation sound will be the stronger, resulting in agreater output at the subtractor junction 356 than at the summing junction 354, thereby causing the gain of the front loudspeakers to be attenuated and the rear loudspeakers to be increased in gain to reproduce with realism the space effect of the concert hall. The time constants of the integrators 366 and 368 are preferably adjustable to permit adjustment to give a pleasing performance; attack times of about one to five milliseconds and decay times of the order of 0.4 seconds are typical.

Although the invention has been described in terms of combining signals in specific proportions, namely, 1.00 and-0.707, and in terms of specific relative phase angles, it is to be understood that minor departures from these specified values may be made without affecting system operation. Thus, although it is convenient to describe and claim the invention in specific terms, it is intended that the claims shall encompass such departures from the stated values.

I claim:

1. Signal decoding apparatus for decoding first and second composite signals L and R respectively containing dominant signals L and R in phase with each other and each including two subdominant signal components L and R in quadrature relationship, with said L and R components in one of said composite signals leading and lagging, respectively, the L and R components in the other composite signal, and with one of the L and R, signals in phase with its respective associated R and L component and the other in phase opposition with the said associated component, said apparatus comprising, in combination:

decoder means including first and second input terminals to which said first and second composite signals are respectively applied and at least two allpass phase-shifting networks connected to respective ones of said input terminals, said phase-shifting networks being operative to shift the phase of one of said composite signals relative to the other by substantially thereby to position the L,, and R components in one of the relatively phase-shifted first and second composite signals either in phase coincidence or in phase opposition with corresponding components in the other relatively phaseshifted composite signal,

signal-combining networks for combining said relatively phase-shifted first and second composite signals to derive third and fourth composite signals respectively containing dominant signal components L and R, which are in phase with each other and each including two subdominant signal components L; and R and 17 means for applying composite signals respectively containing said signal components Lf, R L,, and R,, as predominant components to first, second, third and fourth signal amplitude-modifying means, respectively, for reproduction over four corresponding sound-reproducing devices; control circuit means operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second composite signals contain substantially equal amplitude signal components in phase coincidence or in phase opposition and to produce a first control signal operative when applied to said first and second signal amplitude-modifying means to enhance the gain thereof when said first and second composite signals do not contain said signal components L,, and R;, either in phase or in phase opposition and to produce a second control signal operative when applied to said third and fourth signal amplitude-modifying means to enhance the gain thereof when said third and fourth composite signals do not contain substantially equal signal components either in phase or in phase opposition,

a logic circuit connected to said first and second input terminals operative to compare the sum-and the difference of said first and second composite signals and to produce a third controlsignal when the sum exceeds the difference and to produce a fourth control signal when the difference exceeds I the sum, and

means for combining said first and third control signals and applying the combined control signal to said first and second signal amplitude-modifying means to control the gain thereof, and means for combining said second and fourth control signals and applying the combined control signal to said third and fourth signal amplitude-modifying means to control the gain thereof.

2. Apparatus in accordance with claim 1, wherein said combining means includes circuit means connected to said control circuit means and to said logic circuit and operative to select the larger of said first and said third control signals, and means for applying the larger of said control signals to said signal amplitude-modifying means.

3. Apparatus in accordance with claim 2 wherein said combining means is an OR circuit.

4. Signal-decoding apparatus for decoding first and second composite signals L and R respectively containing dominant signals L, and R; in phase with each other and each including two subdominant signal components L and R in quadrature relationship, with the L,, and R components in one of said composite signals leading and lagging, respectively, the L,, and R components in the other of said composite signals, and with one of the L, and R signals in phase opposition with its respective associated R or L component and the other in phase with the said associated component, the apparatus including:

phase-shifting means operative to shift the phase of one of said composite signals relative to the other.

by substantially 90 thereby to position the L and R components in one of the relatively phaseshifted first and second composite signals either in phase coincidence or in phase opposition with corresponding components in the other;

signal-combining networks for combining said relatively phase-shifted first and second composite signals to derive third and fourth composite signals respectively containing dominant signal components L and R, which are in phase with each other and each including two subdominant components L; and R g I first, second, third and fourth gain control means connected to receive composite signals respectivelycontaining said signal'components L,, R L,, and R, as predominant components;

a control circuit operative in response to said relatively phase-shifted first and second composite signals to produce a first control signal operative when applied to said first and second gain control means to enhance the gain thereof when said first and second composite signals do not contain substantially equal signal components either in phase or in phase opposition and to produce a second control signal operative when applied to said third and fourth gain control means to enhance the gain thereof whenv said third and fourth composite signals do not contain substantially equal signal components either in phase or in phase opposition;

a logic circuit connected to receive and operative to compare the sum and the difference of said first and second composite signals and to produce a third control signal when the sum'exceeds the difference and to produce a fourth control signal when the difference exceeds the sum; and

means for combining the control signals from said control circuit with thecontrol signals from said logic circuit for producing fifth and sixth control signals for application tothe said gain control means to control the gain thereof.

5. Apparatus in accordance with claim 4, wherein said control circuit is operative to produce first and second control signals, and said logic circuit is operative to produce third or fourth control signals depending upon whether the sum exceeds the difference of the first and second composite signals, or vice versa, and wherein said combining means comprises means for adding the third and fourth control signals to the'first and second control signals, respectively.

6. Apparatus in accordance withclaim 4, wherein said combining means includes a circuit connected to said control circuit and to said logiccircuit and operative to select the larger of the control signals produced by the control circuit and the logic circuit, and means for applying the larger of the signals to said gain control means.

7. Apparatus for reproducing on four soundreproducing devices four directional audio information signals respectively designated L R L,, and R contained in first and second composite signals respectively containing to the extent they are present dominant L, and R; component signals, and 'each including to the extent they are present sub-dominant L and R component signals, with said L and R component signals in one of said composite signals in substantially quadrature relationship with the corresponding component signals in the other of the composite signals, the combination comprising:

decoding circuit means including first and second pairs of all-pass phase-shifting networks connected to receive vsaid first and second composite signals, respectively, a first phase-shifting network of each pair being operative to shift the phase of the applied signal by a predetermined reference angle and a second phase-shifting network of each pair being operative to shift the phase of the applied signal by an angle differing from said reference angle by substantially 90,

signal-combining networks for combining the relatively phase-shifted first and second composite signals from the first phase-shifting network of one of said pairs and from the second phase-shifting network of the other of said pairs to derive third and fourth composite signals respectively containing dominant signal components L and R,, which are in phase with each other and each including two subdominant signal components L; and R signal-coupling means connected to receive and operative to couple composite signals respectively containing said signal components L;, R,, L and R as their predominant components to respective ones of first, second, third and fourth soundreproducing devices, said signal-coupling means including signal amplitude-modifying means for separately adjusting the amplitude of the composite signal applied thereto,

a control circuit operative in response to relatively phase-shifted first and second composite signals from the first phase-shifting network of one of said first and second pairs of phase-shifting networks and from the second'phase-shifting network of the other pair to produce a first control signal operative when applied to said first and second, signal amplitude-mo difying means to enhance the gain thereof when said first and second composite signals do not contain substantially equal signal components either in phase or in phase opposition or a second control signal operative when applied to said third and fourth signal amplitude-modifying means to enhance the gain thereof when said third and fourth composite signals do not contain substantially equal signal components either in phase or in phase opposition,

logic circuit connected to receive and operative to compare the sum and thedifference of the first and second composite signals and to produce at least one of third and fourth control signals depending upon whether the sum exceeds the difference, or vice versa, and

means for combining the first or the second control signals from said control circuit with the-third or the fourth control signals, respectively,-from said logic circuit for producing fifth and sixth control signals for application to said first and second and to said third and fourth signal amplitude-modifying means, respectively, to control the transmission characteristic thereof.

8. Apparatus in accordance with claim 7, wherein said last-mentioned means comprises means for adding the third and fourth control signals to the first and second control signals, respectively.

9. Apparatus in accordance with claim 7, wherein said last-mentioned means includes a circuit connected to said control circuit and to said logic circuit and operative to select the larger of the control signals produced by the control circuit and by the logic circuit, and

means for applying the larger of the signals to said sig- 20 nal amplitude-modifying means.

10. Apparatus in accordance with claim 7, wherein said logic circuit includes first and second summing junctions each connected to receive both of said first and second composite signals and operative to produce sum and difference signals, respectively,

first and second logarithmic amplifiers connected to receive and operative to amplify said sum and difference signals, respectively, and

third and fourth summing junctions each connected to receive both of said amplified sum and difference signals andoperative to produce at least one of said third and fourth control signals depending upon whether the sum of the first and second composite signals exceeds the difference of the first and second composite signals, or vice versa.

11. Apparatus for reproducing four individual audio information signals respectively designated L R L and R contained in first and second composite signals respectively containing to the extent they are present dominant L, and R; component signals and each including to the extent they are present subdominant L and R component signals with a phase-shift angle of substantially'90 between said L component signals and between said R component signals, the combination comprising:

decoding circuit means connected to receive said first and second composite signals and operative in response thereto to derive third and fourth composite signals respectively containing predominant L and R component signals and each including sub-dominant L, and R, component signals,

first, second, third and fourth signal amplitudemodifying means respectively connected to receive and operative to couple said first, second, third and fourth composite signals to respective ones of first, second, third and fourth sound-reproducing means,

a logic circuit connected to receive and operative to compare the sum and the difference of said first and second composite signals and to produce a first control signal when the sum exceeds the difference and to produce a second control signal when the difference exceeds the sum, and

means for applying said first control signal to said first and second signal amplitude-modifying means, and means for applying said second control signal to said third and fourth signal amplitude-modifying means.

12. Apparatus in accordance with claim 11, wherein said logic circuit includes first and second summing junctions to each of which both said first and second composite signals are applied and operative to produce sum and difference signals, respectively,

first and second logarithmic amplifiers connected to receive and operative to amplify said sum and difference signals, respectively, and

means connected to receive said amplified sum and difference signals and operative to produce one or the other of said first and second control signals depending upon whether the amplified sum signal exceeds the amplified difference signal, or vice versa. 

1. Signal decoding apparatus for decoding first and second composite signals LT and RT respectively containing dominant signals Lf and Rf in phase with each other and each including two subdominant signal components Lb And Rb in quadrature relationship, with said Lb and Rb components in one of said composite signals leading and lagging, respectively, the Lb and Rb components in the other composite signal, and with one of the Lf and Rf signals in phase with its respective associated Rb and Lb component and the other in phase opposition with the said associated component, said apparatus comprising, in combination: decoder means including first and second input terminals to which said first and second composite signals are respectively applied and at least two all-pass phase-shifting networks connected to respective ones of said input terminals, said phase-shifting networks being operative to shift the phase of one of said composite signals relative to the other by substantially 90* thereby to position the Lb and Rb components in one of the relatively phase-shifted first and second composite signals either in phase coincidence or in phase opposition with corresponding components in the other relatively phase-shifted composite signal, signal-combining networks for combining said relatively phaseshifted first and second composite signals to derive third and fourth composite signals respectively containing dominant signal components Lb and Rb which are in phase with each other and each including two subdominant signal components Lf and Rf, and means for applying composite signals respectively containing said signal components Lf, Rf, Lb and Rb as predominant components to first, second, third and fourth signal amplitudemodifying means, respectively, for reproduction over four corresponding sound-reproducing devices; control circuit means operative in response to at least said relatively phase-shifted first and second composite signals to detect whether said relatively phase-shifted first and second composite signals contain substantially equal amplitude signal components in phase coincidence or in phase opposition and to produce a first control signal operative when applied to said first and second signal amplitude-modifying means to enhance the gain thereof when said first and second composite signals do not contain said signal components Lb and Rb either in phase or in phase opposition and to produce a second control signal operative when applied to said third and fourth signal amplitude-modifying means to enhance the gain thereof when said third and fourth composite signals do not contain substantially equal signal components either in phase or in phase opposition, a logic circuit connected to said first and second input terminals operative to compare the sum and the difference of said first and second composite signals and to produce a third control signal when the sum exceeds the difference and to produce a fourth control signal when the difference exceeds the sum, and means for combining said first and third control signals and applying the combined control signal to said first and second signal amplitude-modifying means to control the gain thereof, and means for combining said second and fourth control signals and applying the combined control signal to said third and fourth signal amplitude-modifying means to control the gain thereof.
 2. Apparatus in accordance with claim 1, wherein said combining means includes circuit means connected to said control circuit means and to said logic circuit and operative to select the larger of said first and said third control signals, and means for applying the larger of said control signals to said signal amplitude-modifying means.
 3. Apparatus in accordance with claim 2 wherein said combining means is an OR circuit.
 4. Signal-decoding apparatus for decoding first and second composite signals LT and RT respectively containing dominant signals Lf and Rf in pHase with each other and each including two subdominant signal components Lb and Rb in quadrature relationship, with the Lb and Rb components in one of said composite signals leading and lagging, respectively, the Lb and Rb components in the other of said composite signals, and with one of the Lf and Rf signals in phase opposition with its respective associated Rb or Lb component and the other in phase with the said associated component, the apparatus including: phase-shifting means operative to shift the phase of one of said composite signals relative to the other by substantially 90* thereby to position the Lb and Rb components in one of the relatively phase-shifted first and second composite signals either in phase coincidence or in phase opposition with corresponding components in the other; signal-combining networks for combining said relatively phase-shifted first and second composite signals to derive third and fourth composite signals respectively containing dominant signal components Lb and Rb which are in phase with each other and each including two subdominant components Lf and Rf; first, second, third and fourth gain control means connected to receive composite signals respectively containing said signal components Lf, Rf, Lb and Rb as predominant components; a control circuit operative in response to said relatively phase-shifted first and second composite signals to produce a first control signal operative when applied to said first and second gain control means to enhance the gain thereof when said first and second composite signals do not contain substantially equal signal components either in phase or in phase opposition and to produce a second control signal operative when applied to said third and fourth gain control means to enhance the gain thereof when said third and fourth composite signals do not contain substantially equal signal components either in phase or in phase opposition; a logic circuit connected to receive and operative to compare the sum and the difference of said first and second composite signals and to produce a third control signal when the sum exceeds the difference and to produce a fourth control signal when the difference exceeds the sum; and means for combining the control signals from said control circuit with the control signals from said logic circuit for producing fifth and sixth control signals for application to the said gain control means to control the gain thereof.
 5. Apparatus in accordance with claim 4, wherein said control circuit is operative to produce first and second control signals, and said logic circuit is operative to produce third or fourth control signals depending upon whether the sum exceeds the difference of the first and second composite signals, or vice versa, and wherein said combining means comprises means for adding the third and fourth control signals to the first and second control signals, respectively.
 6. Apparatus in accordance with claim 4, wherein said combining means includes a circuit connected to said control circuit and to said logic circuit and operative to select the larger of the control signals produced by the control circuit and the logic circuit, and means for applying the larger of the signals to said gain control means.
 7. Apparatus for reproducing on four sound-reproducing devices four directional audio information signals respectively designated Lf, Rf, Lb and Rb contained in first and second composite signals respectively containing to the extent they are present dominant Lf and Rf component signals, and each including to the extent they are present sub-dominant Lb and Rb component signals, with said Lb and Rb component signals in one of said composite Signals in substantially quadrature relationship with the corresponding component signals in the other of the composite signals, the combination comprising: decoding circuit means including first and second pairs of all-pass phase-shifting networks connected to receive said first and second composite signals, respectively, a first phase-shifting network of each pair being operative to shift the phase of the applied signal by a predetermined reference angle and a second phase-shifting network of each pair being operative to shift the phase of the applied signal by an angle differing from said reference angle by substantially 90*, signal-combining networks for combining the relatively phase-shifted first and second composite signals from the first phase-shifting network of one of said pairs and from the second phase-shifting network of the other of said pairs to derive third and fourth composite signals respectively containing dominant signal components Lb and Rb which are in phase with each other and each including two subdominant signal components Lf and Rf, signal-coupling means connected to receive and operative to couple composite signals respectively containing said signal components Lf, Rf, Lb and Rb as their predominant components to respective ones of first, second, third and fourth sound-reproducing devices, said signal-coupling means including signal amplitude-modifying means for separately adjusting the amplitude of the composite signal applied thereto, a control circuit operative in response to relatively phase-shifted first and second composite signals from the first phase-shifting network of one of said first and second pairs of phase-shifting networks and from the second phase-shifting network of the other pair to produce a first control signal operative when applied to said first and second signal amplitude-modifying means to enhance the gain thereof when said first and second composite signals do not contain substantially equal signal components either in phase or in phase opposition or a second control signal operative when applied to said third and fourth signal amplitude-modifying means to enhance the gain thereof when said third and fourth composite signals do not contain substantially equal signal components either in phase or in phase opposition, a logic circuit connected to receive and operative to compare the sum and the difference of the first and second composite signals and to produce at least one of third and fourth control signals depending upon whether the sum exceeds the difference, or vice versa, and means for combining the first or the second control signals from said control circuit with the third or the fourth control signals, respectively, from said logic circuit for producing fifth and sixth control signals for application to said first and second and to said third and fourth signal amplitude-modifying means, respectively, to control the transmission characteristic thereof.
 8. Apparatus in accordance with claim 7, wherein said last-mentioned means comprises means for adding the third and fourth control signals to the first and second control signals, respectively.
 9. Apparatus in accordance with claim 7, wherein said last-mentioned means includes a circuit connected to said control circuit and to said logic circuit and operative to select the larger of the control signals produced by the control circuit and by the logic circuit, and means for applying the larger of the signals to said signal amplitude-modifying means.
 10. Apparatus in accordance with claim 7, wherein said logic circuit includes first and second summing junctions each connected to receive both of said first and second composite signals and operative to produce sum and difference signals, respectively, first and second logarithmic amplifiers connected to receive and operative to amplify said sum and difference signals, respectively, and third and fourth summing junctions each connected to receive both of said amplified sum and difference signals and operative to produce at least one of said third and fourth control signals depending upon whether the sum of the first and second composite signals exceeds the difference of the first and second composite signals, or vice versa.
 11. Apparatus for reproducing four individual audio information signals respectively designated Lf, Rf, Lb and Rb contained in first and second composite signals respectively containing to the extent they are present dominant Lf and Rf component signals and each including to the extent they are present sub-dominant Lb and Rb component signals with a phase-shift angle of substantially 90* between said Lb component signals and between said Rb component signals, the combination comprising: decoding circuit means connected to receive said first and second composite signals and operative in response thereto to derive third and fourth composite signals respectively containing predominant Lb and Rb component signals and each including sub-dominant Lf and Rf component signals, first, second, third and fourth signal amplitude-modifying means respectively connected to receive and operative to couple said first, second, third and fourth composite signals to respective ones of first, second, third and fourth sound-reproducing means, a logic circuit connected to receive and operative to compare the sum and the difference of said first and second composite signals and to produce a first control signal when the sum exceeds the difference and to produce a second control signal when the difference exceeds the sum, and means for applying said first control signal to said first and second signal amplitude-modifying means, and means for applying said second control signal to said third and fourth signal amplitude-modifying means.
 12. Apparatus in accordance with claim 11, wherein said logic circuit includes first and second summing junctions to each of which both said first and second composite signals are applied and operative to produce sum and difference signals, respectively, first and second logarithmic amplifiers connected to receive and operative to amplify said sum and difference signals, respectively, and means connected to receive said amplified sum and difference signals and operative to produce one or the other of said first and second control signals depending upon whether the amplified sum signal exceeds the amplified difference signal, or vice versa. 